The Fabric of Reality
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The Fabric of Reality is a book by physicist David Deutsch written in 1997. It expands upon his views of quantum mechanics and its implications for understanding reality. This interpretation, which he calls the multiverse hypothesis, is one of a four-strand Theory of Everything (TOE). The four strands are:
- Hugh Everett's many-worlds interpretation of quantum physics, "The first and most important of the four strands".
- Karl Popper's epistemology, especially its anti-inductivism and its requiring a realist (non-instrumental) interpretation of scientific theories, and its emphasis on taking seriously those bold conjectures that resist falsification.
- Alan Turing's theory of computation especially as developed in Deutsch's "Turing principle", Turing's Universal Turing machine being replaced by Deutsch's universal quantum computer. ("The theory of computation is now the quantum theory of computation.")
- Richard Dawkins's refinement of Darwinian evolutionary theory and the modern evolutionary synthesis, especially the ideas of replicator and meme as they integrate with Popperian problem-solving (the epistemological strand).
His theory of everything is (weakly) emergentist rather than reductive. It aims not at the reduction of everything to particle physics, but rather mutual support among multiverse, computational, epistemological, and evolutionary principles.
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The many worlds interpretation of quantum mechanics 
According to the many worlds interpretation of quantum mechanics, the total of physical reality is actually a multiverse, consisting of an infinity of universes. Deutsch argues for it on four main grounds:
- The situation of having multiple interpretations of quantum mechanics is anomalous. People should settle for the most straightforward (realistic) interpretation, which is, according to Deutsch, Many Worlds.
- Only a Many Worlds approach can explain the additional power of quantum computers compared with classical computers.
- Only Many Worlds can explain quantum superposition effects, such as those observed in single-particle double slit experiments.
- Most other approaches — the Bohm interpretation, the Transactional interpretation — are Many Worlds in disguise or denial.
- The interpretational debate is not unprecedented. There was a debate about the nature of gravity and action-at-a-distance in Newtonian physics. There is a debate about the ontology of space in relativity theory. There is even a debate about the ontology of mathematics.
- Quantum computers are still Turing-complete machines. Given a "black box" computer, it is impossible to determine whether the inner workings are quantum or classical. Quantum computing allows more computational power to be extracted from a given set of physical resources. It is debatable whether multiple quasi-classical worlds are a better ontological model of quantum computing than a single quantum world. (A possible riposte is at p. 217 of Fabric, where Deutsch derives the limitations of a Turing machine from a consideration about the resources available to execute Shor's algorithm. Given his thoroughgoing physicalism, he is apparently "within his argument" to dismiss the powers of an abstract black-box Turing machine.)
- To interpret a superposed photon as being in two different "worlds" requires an idiosyncratic interpretation of "world". The superposition is only detectable through interference effects, which in turn means the two photons (or two states of the photon, depending on interpretation) are coherent. Most Many Worlds take the view that a quantum state does not count as a "world" until it decoheres. (But compare Lev Vaidman's discussion of worlds in MWI at Stanford Encyclopedia of Philosophy. which implies that differences among MWI theorists about the use of the term 'world' are "only semantic".)
- An interesting point which perhaps relies on the "coherent worlds" approach mentioned above.
The Church–Turing thesis 
This emergentist posture allows Deutsch to do some serious work with the Church-Turing thesis (or "Turing principle" as he calls it), which is fundamental in theoretical computer science. In the strong form he favors it implies that a universal quantum computer, capable of rendering any physically possible environment, actually exists near the end of spacetime in every universe and is maintained by sentient beings with the knowledge required to increase its memory, computing cycles, and energy supply. In this he follows Frank Tipler in The Physics of Immortality, though he emphasizes the scientific component of Tipler's Omega Point hypothesis, the component that is justified by Popperian epistemology as implied by our best science. He is much less sympathetic to the non-scientific component, which provides rational reconstructions for traditional theological categories such as God, omniscience, omnipresence, benevolence, creation, and so on.
The strong form of the Turing principle rests on a mathematical argument favoring a universal quantum computer in all universes over such a computer in one universe, and either of these over such a computer in, say, 17 universes. So a weaker form of the Turing principle would commit to one universe. Also the reasoning for 'either one or all' presupposes a Big Crunch cosmology in order to generate the energy for the required computing cycles. If the universe instead expands forever, the Turing principle would have to take a weaker form, implying the existence of a more or less remote approximation to a universal quantum computer.
Deutsch's Turing principle is also sometimes called the Church-Turing-Deutsch principle by those who question whether Turing's work on the foundations of computing was aiming to disclose what could be computed tractably "in nature". More conservative readings of Turing view him as concerned with what could be computed "by human computers", i.e. human mathematicians. On this reading Turing didn't aim at foundations for computing that provided tractability, because an algorithm might be calculable "by human computers" but without the speed to compute tractably what happens "in nature". (The humans might take a very long time to finish their computations.) There are tractability issues when, for instance, factoring and decryption problems are attacked with Turing-machine or classical-computation methods, problems that seem to be resolved by quantum-computing techniques such as Shor's algorithm, which takes advantage of the superposition of states in qubits to calculate "all at once" what a Turing machine would calculate serially. Turing universality isn't universal enough, Deutsch thinks. Turing's abstract computer needs to be replaced by the actual, physical, universal quantum computer derived from the Church-Turing (-Deutsch) principle.
That principle is also sometimes called the Matrix principle, because Deutsch's conception of virtual reality figures in its statement: "It is possible to build a virtual reality generator whose repertoire includes every physically possible environment." Some cognitive psychologists[who?] think that Deutsch's view of the brain as a virtual-reality generating computer, adequate to rendering a humanly experienced environment, affords a sufficiently robust account of subjective experience or qualia, one consistent with a view of the mind/brain as a computer, to break down the impasse between qualophobes and qualophiles. (However, it is arguable that his approach confuses qualia with mental representations). With considerable thrust concentrated position it is theorised that this can alter the fabric of a questioned reality. A virtual-reality generator consists of an image generator to provide the subject with perceptual content from the several sensory modalities, perhaps in the forms of transducers connected directly to afferent nerves by use of neural implants, and a program to handle interaction between the subject's choices and the virtual environment. Nearer to the Omega Point this transhuman enhanced-biology scenario gives way to a posthuman condition, because biology becomes untenable. Gravitational shearing and other extreme forces call for more durable substrates for human psychology. The brain is replaced by sturdy computational equivalents in virtual realities, protected from the Big Crunch and pushed in the final moments by unlimited computational cycles affording their posthuman residents the subjective experience of immortality.
Quantum computers and proof theory 
A quantum computer farms out computing problems to other universes in order to achieve tractability for solutions that otherwise get bogged down by exponentially increasing demands for more time and other computational resources. The apparent need on a realist conception of science to posit such collaboration inspires a pugnacious comment from Deutsch: "To those who still cling to a single-universe world-view, I issue this challenge: explain how Shor's algorithm works." The challenge is meant to imply that a Turing machine is incapable in principle of doing what a quantum computer can do, since the latter's operations in executing Shor's algorithm require computational resources from other worlds. And generally, a quantum computer's operations include computational steps in other worlds that are not present in any Turing-machine's tape (in this world). Deutsch thinks this has implications for proof theory, which must abandon the Cartesian model of an inspectable list of premises leading to a conclusion, in favor of a model of a process in which the relationship between premises and conclusion may be mediated by computations that are not inspectable (in this world).
Counterfactual inferences and modal realism 
Another important theme in the book is that basic ideas about the universe are either vindicated or undermined by the multiverse hypothesis. For instance, counterfactual conditionals refer to nearby parallel worlds when they stipulate what a thing would do under conditions that do not actually obtain. One-worlders implicitly collapse what things can do into what they actually do (in this world). Consider a coin toss. The identical worlds in which I (copies of me) see it spinning become branched; in fifty-percent of those worlds versions of me see 'heads', and in fifty percent they see 'tails'. This actual distribution of worlds is what licenses the inference, about this world, that if the coin hadn't turned up 'heads' it would have turned up 'tails'. Instead of its being a basic fact that my observing 'heads' collapses probabilities into an actual outcome of the coin toss, those probabilities are grounded in actual universes in which both outcomes are represented. This is sometimes referred to as the difference between "collapse theories" (e.g., the Copenhagen interpretation) and no-collapse theories (e.g., many-worlds or the multiverse interpretation).
Deutsch acknowledges a kindred spirit in the philosopher David Lewis, whose modal realism handles counterfactuals in a similar fashion. He takes Lewis to have "postulated the existence of a multiverse for philosophical reasons alone." This is a contentious claim, since Lewis's realism about parallel worlds extends to worlds that are not physically possible, such as the world where Harry Potter was schooled at Hogwarts, whereas the parallel worlds in Deutsch's multiverse comprise all and only physically possible worlds. The worlds of the multiverse are governed by the same natural laws. Also Lewis's possible worlds are disjoint, whereas Deutsch's parallel worlds interact through interference[disambiguation needed]. On the other hand, Lewis recognizes overlapping worlds as a theoretical possibility, and Deutsch's universal quantum computer can render Harry Potter worlds to any desired degree of accuracy.
Knowledge and life 
Knowledge is a trans-universe structure, as one might expect because knowledge supports counterfactual implications, as revealed for instance in Robert Nozick's tracking account of knowledge. Nearby parallel worlds are united by a common history of knowledge acquisition, spelled out in broadly Popperian terms. The resulting epistemological niche lends stability and reliability to knowledge in each universe. Life is a similar trans-universe structure, molded by natural selection rather than rational criticism. What distinguishes genuine replicating DNA from junk DNA is that the former but not the latter is representative of a niche of replicators that extends across worlds. Indeed personal identity is inseparable from such a niche, which Deutsch picks out with the word "copies". A person is a set of copies in nearby parallel worlds. This comes out in his analysis of free will: I could have chosen otherwise is analysed as Other copies of me chose otherwise. And in the dénouement to a dramatic chapter that rehearses interference experiments from a multiverse viewpoint, he writes of his copies, "Many of those Davids are at this moment writing these very words. Some are putting it better. Others have gone for a cup of tea."
Time, causation and free will 
Not only are persons spread out through worlds, but they, like everything else, are quantized through time in any given world. Time is a series of moments, and a person who exists at a moment exists there forever in four-dimensional spacetime, rather than being transformed continuously through the flow of time. Such change and flow are mythical, Deutsch argues. The argument doesn't strictly require the multiverse hypothesis, because deterministic physics since Newton has implied that the openness of the future is an illusion, and consequently that free will is an illusion. (This conclusion could be avoided by adopting compatibilism. Also, collapse interpretations of quantum mechanics imply both indeterminism and an open future).
What the multiverse adds to a block time theory is an attenuated account of common sense's ideas of causation and free will. Although an effect can't be changed by its cause, the counterfactuals that causal statements support are true. If the cause hadn't occurred, the effect would not have occurred. For the multiverse, which is "to a first approximation" a very large number of co-existing and slightly interacting spacetimes, includes universes in which the cause doesn't occur and its effect doesn't occur. And although the "me-copy" in this spacetime could not have done otherwise, there are me-copies in other worlds that actually do otherwise (thus, the common-sense idea that, in choosing one course of action, one refrains from another, is not retained). There is a branching of these me-copies that validates my sense that my future is open, in contrast to spacetime physics. However, the open future of common sense is a myth. As defined by the Darwinist framework, there is no flow of time dividing the actualities of the past from the unactualized potentialities of the future.
An intellectual descendant of David Hume via the paternity of Popper, Deutsch is not only a critic of induction but also a Humean about causation, to the degree that he rejects the idea of a causal power effecting a change, in favor of construing it as a multiverse regularity. So A causes B means something like After A-copies occur in many nearby parallel worlds, including the one in this world, B-copies occur. This regularity supports counterfactuals that accompany true causal claims, such as If A hadn't happened, B would not have taken place. There are affinities to Hume's constant-conjunction understanding of causation and Popper's deductive-nomological account.
Time and personal identity 
Since "other times are just special cases of other universes" (an idea that has been much expanded by Julian Barbour), the temporal granularity of personhood through time is a special case of being spread out through worlds. In addition to one's identically time-stamped copies at a moment across parallel worlds transversely, there are the differently time-stamped copies across parallel worlds longitudinally, linked by natural law so as to give the individual's experience of one world and a continuous self. The implications for the theory of personal identity are not yet clear, but Derek Parfit's Reductionist view seems to be favored: The concept of personal identity ceases to apply when branching is taken into account, but branching maintains what's important about personal identity, such as psychological continuities having to do with memory, desire, character, and so forth. Another possibility is that Robert Nozick's Closest-Continuer theory could be modified so as to track closeness transversely as well as longitudinally. The tracked slices of "me-copies" would be the continuing person. Deutsch would seem to favor some such approach. There are "multiple identical copies" of me in the multiverse. Which one am I? Deutsch answers, "I am, of course, all of them." (The Parfitian answer would be, "The concept of personal identity doesn't apply.") Copies need not be strictly identical in the sense of the identity of indiscernibles relativized to universes: All of my copies see a coin spinning in a coin toss, but an instant later half my copies see 'heads' come up, the other half see 'tails'. A distinction between copies, versions, and variants is at work here. Variants of me need not see the spinning coin. Versions of me see it though some of them see 'heads' and some 'tails'. The multiple identical copies of me all see the spinning coin.
Time travel 
The possibility of future-directed time travel is assured by Einstein's special theory of relativity, which says that an observer who accelerates or decelerates will experience less time than an observer who is at rest or in uniform motion. This time dilation could make an astronaut's flight very short and the duration on Earth very long, but such a trip to Earth's future would be irreversible as "no amount of time dilation can allow a spaceship to return from a flight before it took off". As for past-directed time travel, it is possible as a sort of sidestep from one universe to another, requiring a path between the two universes that is "hard-wired" into the structure of the multiverse. Whether such paths exist or not is an unresolved empirical question. If they were to exist and were to allow macro-objects like human beings to traverse them, time travel could occur without the grandfather paradox, because the time traveler would go to a point prior to the branching between his "home" universe and the universe in which he (the copy of him in the "away" universe) kills his grandfather (grandfather-copy).
The multiverse hypothesis alone doesn't avoid the knowledge paradox, in which the time traveler goes to a point where he gives the collected works of Shakespeare to a hack writer, who seems to get "knowledge for free" that he uses to become the celebrated Shakespeare in that world. The Popperian epistemological strand of Deutsch's four-strand theory is invoked at this point. Just as life is understood as a trans-universe structure that is physically possible only through processes of natural selection, so too knowledge is understood as such a structure that is physically possible only through processes of rational problem-solving. Knowledge for free isn't possible in the multiverse when the multiverse is understood through Deutsch's emergentist Theory of Everything.
See also 
- When Shor's algorithm has factorized a number, using 10500 or so times the computational resources that can be seen to be present, where was the number factorized? There are only about 1080 atoms in the entire visible universe, an utterly minuscule number compared with 10500. So if the visible universe were the extent of physical reality, physical reality would not even remotely contain the resources required to factorize such a large number. Who did factorize it, then? How, and where, was the computation performed?
- Vaidman presents MWI as consisting of two parts, (i) a mathematical theory which yields evolution in time of the quantum state of the (single) universe, and (i) a prescription which sets up a correspondence between the quantum state of the Universe and our experiences. The concept of 'world' in MWI belongs to part (ii) of the theory and is not rigorously defined. Vaidman writes, "Part (i) is essentially summarized by the Schrödinger equation or its relativistic generalization. It is a rigorous mathematical theory and is not problematic philosophically. Part (ii) involves "our experiences" which do not have a rigorous definition. An additional difficulty in setting up (ii) follows from the fact that human languages were developed at a time when people did not suspect the existence of parallel worlds. This, however, is only a semantic problem."
- Stanford Encyclopedia of Philosophy on the Church-Turing thesis.
- Papineau, D. 'David Lewis and Schrödinger's Cat'.
- Truth-Tracking and the Problem of Reflexive Knowledge
- Wallace, D. Everett and Structure.
- The Fabric of Reality paperback edition: ISBN 0-14-027541-X